Method of making shielded conductors



B. c. DAVIS ETAL METHOD OF MAKING SHIELDED CONDUCTORS Filed oct. 4, 196s@zz/22e,

f /f 1/ //V/// Nov. 21, 1967 United States Patent 3,353,260 METHOD FMAKING SHIELDED CGNDUCTORS Bayard C. Davis, Lombard, Alvin Singer,Glencoe, and Milford 'I'. Chapman, Sir., Addison, Ill., assignors, bydirect and mesne assignments, to Continental Sensing, Inc., MelrosePark, Ill., a corporation of Illinois Filed Oct. 4, 1963, Ser. No.313,747 4 Claims. (Cl. Z9- 573) This invention relates to a method ofmaking shielded conductors and more particularly to an improved processfor making metal sheathed thermocouples.

As is well known, metal sheathed thermocouples are generally comprisedof three basic components, namely an outer metallic sheath, suitableinsulating material, and one or more sensing wires that are surroundedand suitably maintained in spaced relation by the insulating materialand confined within the sheath. For some time a conflict has existedbetween those skilled in the art as to which of several techniquesprovides the best answer to the manufacture of such thermocouples in anydesired length. The basic problem to be faced in connection with theproduction of metal sheathed thermocouples is the provision of athermocouple structure which is characterized by good thermal andelectrical characteristics, relatively unaffected by moisturepenetration, free from gas residue and possessing a metallic sheathwhich is structurally strong and not prone to rupture.

It is a prime object of the present invention to provide an improvedmethod of making shielded conductors.

It is a further object of the present invention to provide an improvedprocess for manufacturing metal sheathed thermocouples in any desirablelength, which thermocouples possess excellent thermal and electricalproperties.

An additional object of the present invention is to provide a processfor manufacturing metal sheathed thermocouples whereby the resultingproduct is substantially unaffected by adverse environmental conditions,is characterized by substantially uniform insulation density throughoutthe length of the thermocouple, and possesses a metallic sheath which isneither rendered susceptible to rupture nor otherwise structurallyweakened as a result of the manufacturing process.

Still another obiect of the present invention is to provide an improvedmethod of making metal sheathed thermocouples whereby and as anadditional step of the basic process the sensing wires surrounded by thedensely compacted insulating material and confined within thestructurally stable sheath are selectively twisted to provide athermocouple that effectively eliminates the problem of extraneoussignal pickup which has heretofore severely limited the utility ofthermocouples under various operating conditions.

Other objects and advantages of the presentV invention will becomeapparent from the following description wherein a general presentationof the overall process is followed by a detailed description of thepreferred steps and a preferred sequence of Iperforming the steps of theprocess as reflected in the accompanying drawing wherein:

FIGURE l depicts the first drawing step involved in the process of thepresent invention;

FIGURE lA is a left end view of the assembly being processed as shown inFIGURE l;

FIGURE 1B is a right end view of the assembly being processed as shownin FIGURE 1;

FIGURE 2 illustrates the swaging step involved in the process of thepresent invention which follows the drawing step depicted in FIGURE l;

FIGURE 2A is a right end view of the assembly being processed as shownin FIGURE 2;

ICC

FIGURE 3 illustrates the second drawing step involved in the process ofthe present invention and further illustrates means for effecting atwisting operation in accordance with one preferred embodiment of theinvention;

FIGURE 3A is a cross-sectional view taken along the line 3A-3A in FIGURE3;

FIGURE 3B is a cross-sectional View taken along the line 3B-3B in FIGURE3; and

FIGURE 4 depicts a metal sheathed thermocouple structure processed inaccordance with the present invention.

In general, the improved process of the present invention is initiatedby operations which yield a thermocouple assembly of the desired length.In this assembly, the sensing wires as surrounded and held in place bythe insulating material, and this subassernbly is in turn relativelyloosely tted within a tubular metallic element that serves as the sheathfor the completed thermocouple. This thermocouple assembly is thensubjected to a suitable number of drawing and swaging operations untilthe thermocouple has been formed.

As a result of these sequentially performed operations, the insulatingmaterial is densely and uniformly cornpacted about the sensing wires,but not so as to damage or otherwise harm the wires. The metallic sheathis worked so that it intimately engages and contines the insulatingmaterial wherein the sensing wires are maintained in uniform spacedrelation. As a result, a substantially uniform insulation pack densityis realized within the thermocouple, and the exterior sheath has asubstantially uniform cross section and is not susceptible to structuralfailure such as rupturing.

It is often desired to produce a metal sheathed thermocouple wherein thesensing wires are suitably twisted so as to effectively eliminate thepickup of extraneous signals which are encountered under certainoperating conditions. To this end and in accordance with the presentinvention, the thermocouple can, if desired, be further worked during atleast one of the drawing Operations to uniformly twist the sensing wiresabout each other and in spaced apart relation along the entire length ofthe thermocouple.

More specifically, the process for making metallic sheathedthermocouples in `accordance with the present invention is initiated byselectively feeding, for example, two or more sensing wires throughsuitable apertures in structurally stable ceramic insulator strips. Ineffect, the insulator strips, which may be of variable length and formedof substantially pure ceramic materials such as alumina and magnesiaheld together by a suitable bonding agent, are strung onto the sensingwires so that the entire lengths of the wires are substantiallyencompassed by the strips. Inasmuch as the sensing wires, which may beformed of Alumel and Chromel, are confined within apertures provided inthe insulator strips, these `wires are maintained in substantiallyuniform spaced apart parallel relationship.

This Stringing of the insulator strips onto the sensing Wires yields asubassernbly which is then ready for insertion into a metallic tube (eg.surgically clean stainless steel) that will serve as the sheath for thecompleted thermocouple. In this connection, the insulator strips, whichare susceptible to handling without breaking and further susceptible tobeing returned to a powdered condition upon the application of forcethereto, have an outside diameter which is somewhat less than the insidediameter of the tubular element that is formed into the metallic sheath(FIG. 1A). Accordingly, the strung strips and sensing wires are readilyinsertable Within the metallic tube. When the assembly has beeninserted, a first of the actual thermocouple working operations iscommenced.

The initial working operation performed on the subassembly is thedrawing of the subassembly through a suitable die arrangement (FIG. l)This step of the process is effected by engaging one end of the assemblyafter it has been initially inserted in a stationary die structure andthereafter advancing the remaining portion of the assembly through thedie at a preselected rate.

As a result of this operation, the sheath is drawn into contact with thestrung insulator strips so that the strips are confined in generallyfixed relation within the sheath (FIG. 1B), although the forces impartedto the sensing wires confined within the insulator strips are minimaland although the cross section of the sheath is only slightly reduced.Moreover, this initial drawing operation insures that all of the stripsare brought into mating contact with those adjacent thereto so that acontinuous covering of insulation is provided for the sensing wires. Ashereinafter described in detail, this leads to substantially uniformpack density in the resultant thermocouple subsequent to the otherforming operations.

After this initial drawing operation is completed, the partially workedassembly is advanced through a second, swaging die structure in a mannersimilar to that described above (FiG. 2). This second die structureeffects a swaging operation to within approximately .001 inch of thedesired size of the finished thermocouple. As a result of the swagingoperation, the cross section of the assembly is reduced although littleaxial elongation is experienced.

Since the initial drawing operation caused the adjacent insulator stripsto be drawn into a continuous fixed array, the compacting of theinsulator strips caused by the swaging yields a powdered insulatingmaterial having a substantially uniform pack density throughout thelength of the thermocouple `(FIG. 2A). The compacting of the insulatorstrips and the reduction of the cross section of the sheath does notdisturb the substantially uniform spaced apart relation of the sensingwires that are, as a result of this operation, surrounded by thepowdered insulation.

Any discontinuities, indentations and the like left in the exteriorsheath of the thermocouple as a result of the swaging operation areeliminated as a result of the next step of the process of the presentinvention. That is, the once-drawn and once-swaged thermocouple issimilarly advanced through still another die structure, and the entirethermocouple is drawn so that the outside diameter is reduced to thedesired finished size (FIG. 3). This finishing operation eliminates anyindentations and/or bulges present in the assemblies and the resultingthermocouple has a diameter which is substantially free of variance fromend to end, notwithstanding length.

In a copending application of Messrs. Davis, Singer, and Chapman, Ser.No. 282,406, which was filed on May 22, 1963 (now United States LettersPatent No. 3,205,- 296), a metallic sheathed thermocouple is disclosedand claimed wherein the sensing wires are suitably twisted so that thepickup of extraneous signals thereby is substantially eliminated. Athermocouple structure as contemplated by this copending application canbe readily fabricated pursuant to the process described above. However,incident to the final drawing operation, the entire assembly is furtherworked to yield the desired twisted and spaced apart sensing wires.

In this connection, as the once-drawn and once-swaged thermocoupleassembly is advanced through the final die structure (FIG. 3), asuitable rotary means is brought into engagement with a trailing endportion of the assembly (i.e., a portion of the assembly on the side ofthe die structure away from the path of advancing travel). This suitablerotary means travels with the advancing thermocouple and imparts rotaryor twisting motion thereto. The rotary or twisting rate is dictated inpart by the speed at which the assembly is passed through the final diestructure and by the number of twists per inch required in the completedthermocouple to achieve the elimination of extraneous signals.

The rotary or twisting motion imparted to the thermocouple produces acoaction of forces at the die structure which results in the sensingwires being uniformly twisted and yet maintained in suitable spacedapart relation within the surrounding compacted insulating material(FIGS. 3A and 3B). However, the coaction of forces produced at the diestructure and the sheath working operation carried out by the diestructure are such that the sheathed thermocouple emanating from the diedoes not possess any unusual surface abnormalities, notwithstanding thetwisting of the sensing wires which is concomitantly effected.

It should be understood that the foregoing steps of the process can beeffected by employing any number of suitable cooperating die andassembly advancing instrumentalities. However, the details of suchstructures do not form a part of the invention and will not be dealtwith. Suf-lice it to say that one skilled in the art can conceive of anynumber of suitable arrangements for effecting the operations to achievethe production of a metal sheathed thermocouple which is relatively freefrom any undesirable structural, thermal, or electrical defects andwhich may or may not include one or more pairs of twisted sensing wires.

lt should be further understood that the foregoing is merelyillustrative of one preferred embodiment of the process of the presentinvention. Various modifications of the preferred steps described abovecan be effected without deviating from the invention, various featuresof which are set forth in the accompanying claims.

What is claimed is:

1. A process for producing metal sheathed thermocouple conductors in anydesired length; which process comprises the sequential steps ofpreparing an assembly of sensing wires, insulation material, andmetallic sheathing whereby said sensing wires are supported withindiscrete strips of said insulation material and are arranged insubstantially uniform spaced relation along the entire length of theassembly and whereby said wires and surrounding insulation material areloosely supported within said sheath;

urging said strips into contacting relation with each other and saidsheath by subjecting said assembly to an initial drawing operationthrough a stationary reducing die means;

reducing the cross section of the drawn assembly to approximately thedesired cross section of the completed thermocouple conductor butsubstantially without elongation of said assembly, and at the same timecrushing the insulator strips and loosely compacting the insulationmaterial about said spaced apart sensing wires supported therein bypassing the assembly through a swaging die means;

and finally drawing said drawn and swaged assembly through a secondstationary die means to uniformly reduce the cross section of theassembly to the desired final cross section and further compact saidinsulation material about said wires so that said wires are fixedlymaintained in suitably spaced relation throughout the entire length ofthe completed thermocouple conductor.

2. The process defined in claim 1 wherein rotary motion is imparted tosaid assembly concomitantly with said final drawing operation so thatsaid sensing wires are twisted about each other in uniform spaced-apartrelation throughout the entire length of the completed thermocoupleconductor.

3. A process for producing metal sheathed thermocouple conductors in anydesired length by working an assembly of sensing wires, strips ofinsulation material, and metallic sheathing wherein said sensing wiresare supported within the discrete strips of insulation material insubstantially uniform spaced-apart relation and said wires and strips ofinsulation material are loosely supported within said sheath;

5 6 which process comprises urging said strips into conare twisted abouteach other in uniform spaced-apart tacting relation with each other andsaid sheath by relation throughout the entire length of the completedsubjecting said assembly to an initial drawing operathermocoupleconductor. tion through a stationary reducing die means; reducing thecross section of the drawn assembly to 5 References Cited approximatelythe desired cross section of the completed thermocouple conductor butsubstantially with- UNITED STATES PATENTS out elongation of saidassembly, and at the same Re, 18,272 12/1931 Frahm 29 477 time crushingthe insulator stri-ps and loosely com- 1,157,916 10/1915 Wentworth 33,8238 pacting the insulating material about said spaced l0 1,523,4341/1925 Lightfoot 29 155,65

apart sensing wires supported therein by passing the 2,703,419 3 /1955Barth 10 152 assemblythroughaswaging die means; 2,808,492 10/1957 Yohe29-155.63 X and finally drawing said drawn and swaged assembly 2,851,5719/1958 Pearce 29 155 63 X through a second stationary die means touniformly 3,065,286 11/ 1962 Connell,

reduce the cross section of the assembly to the de- 15 3,096,577 7 /1963Carlson 29 501 X sired nal cross section and further -compact said3,223,878 12/1965 Todd 29 419 X insulation material about said wires sothat said wires are xedly maintained in suitably spaced re- FOREIGNPATENTS lation throughout the entire length of the completed 854,57011/1960 Great Britain.

thermocouple conductor. 20

4. The process defined in claim 3 wherein rotary rno- JOHNF CAMPBELLExaminer tion is imparted to said assembly concomitantly with said finaldrawing operation so that said sensing wires WILLIAM I, BROOKS, PrimaryExaminer,

1. A PROCESS FOR PRODUCING METAL SHEATHED THERMOCOUPLED CONDUCTORS INANY DESIRED LENGTH; WHICH PROCESS COMPRISES THE SEQUENTIAL STEPS OFPREPARING AN ASSEMBLY OF SENSING WIRES, INSULATION MATERIAL, ANDMETALLIC SHEATING WHEREBY SAID SENSING WIRES ARE SUPPORTED WITHINDISCRETE STRIPS OF SAID INSULATION MATERIAL AND ARE ARRANGED INSUBSTANTIALLY UNIFORM SPACED RELATION ALONG THE ENTIRE LENGTH OF THEASSEMBLY AND WHEREBY SAID WIRES AND SURROUNDING INSULATION MATERIAL ARELOOSELY SUPPORTED WITHIN SAID SHEATH; URGING SAID STRIPS INTO CONTACTINGRELATION WITH EACH OTHER AND SAID SHEATH BY SUBJECTED SAID ASSEMBLY TOAN INITIAL DRAWING OPERATION THROUGH A STATIONARY REDUCING DIE MEANS;REDUCING THE CROSS SECTION OF THE DRAWN ASSEMBLY TO APPROXIMATELY THEDESIRED CROSS SECTION OF THE COMPLETED THERMOCOUPLE CONDUCTOR BUTSUBSTANTIALLY WITHOUT ELONGATION OF SAID ASSEMBLY, AND AT THE SAME